package org.apache.lucene.index;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.util.Collection;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.ArrayList;
import java.util.Map;
import java.util.Set;
import java.util.Map.Entry;
import java.io.IOException;

import org.apache.lucene.store.Directory;

/**
 * An {@link IndexDeletionPolicy} that wraps around any other
 * {@link IndexDeletionPolicy} and adds the ability to hold and later release
 * snapshots of an index. While a snapshot is held, the {@link IndexWriter} will
 * not remove any files associated with it even if the index is otherwise being
 * actively, arbitrarily changed. Because we wrap another arbitrary
 * {@link IndexDeletionPolicy}, this gives you the freedom to continue using
 * whatever {@link IndexDeletionPolicy} you would normally want to use with your
 * index.
 * 
 * <p>
 * This class maintains all snapshots in-memory, and so the information is not
 * persisted and not protected against system failures. If persistency is
 * important, you can use {@link PersistentSnapshotDeletionPolicy} (or your own
 * extension) and when creating a new instance of this deletion policy, pass the
 * persistent snapshots information to
 * {@link #SnapshotDeletionPolicy(IndexDeletionPolicy, Map)}.
 * 
 * @lucene.experimental
 */
public class SnapshotDeletionPolicy implements IndexDeletionPolicy {

    /** Holds a Snapshot's information. */
    private static class SnapshotInfo {
        String id;
        String segmentsFileName;
        IndexCommit commit;

        public SnapshotInfo(String id, String segmentsFileName, IndexCommit commit) {
            this.id = id;
            this.segmentsFileName = segmentsFileName;
            this.commit = commit;
        }

        @Override
        public String toString() {
            return id + " : " + segmentsFileName;
        }
    }

    protected class SnapshotCommitPoint extends IndexCommit {
        protected IndexCommit cp;

        protected SnapshotCommitPoint(IndexCommit cp) {
            this.cp = cp;
        }

        @Override
        public String toString() {
            return "SnapshotDeletionPolicy.SnapshotCommitPoint(" + cp + ")";
        }

        /**
         * Returns true if this segment can be deleted. The default implementation
         * returns false if this segment is currently held as snapshot.
         */
        protected boolean shouldDelete(String segmentsFileName) {
            return !segmentsFileToIDs.containsKey(segmentsFileName);
        }

        @Override
        public void delete() {
            synchronized (SnapshotDeletionPolicy.this) {
                // Suppress the delete request if this commit point is
                // currently snapshotted.
                if (shouldDelete(getSegmentsFileName())) {
                    cp.delete();
                }
            }
        }

        @Override
        public Directory getDirectory() {
            return cp.getDirectory();
        }

        @Override
        public Collection<String> getFileNames() throws IOException {
            return cp.getFileNames();
        }

        @Override
        public long getGeneration() {
            return cp.getGeneration();
        }

        @Override
        public String getSegmentsFileName() {
            return cp.getSegmentsFileName();
        }

        @Override
        public Map<String, String> getUserData() throws IOException {
            return cp.getUserData();
        }

        @Override
        public long getVersion() {
            return cp.getVersion();
        }

        @Override
        public boolean isDeleted() {
            return cp.isDeleted();
        }

        @Override
        public int getSegmentCount() {
            return cp.getSegmentCount();
        }
    }

    /** Snapshots info */
    private Map<String, SnapshotInfo> idToSnapshot = new HashMap<String, SnapshotInfo>();

    // multiple IDs could point to the same commit point (segments file name)
    private Map<String, Set<String>> segmentsFileToIDs = new HashMap<String, Set<String>>();

    private IndexDeletionPolicy primary;
    protected IndexCommit lastCommit;

    public SnapshotDeletionPolicy(IndexDeletionPolicy primary) {
        this.primary = primary;
    }

    /**
     * {@link SnapshotDeletionPolicy} wraps another {@link IndexDeletionPolicy} to
     * enable flexible snapshotting.
     * 
     * @param primary
     *          the {@link IndexDeletionPolicy} that is used on non-snapshotted
     *          commits. Snapshotted commits, are not deleted until explicitly
     *          released via {@link #release(String)}
     * @param snapshotsInfo
     *          A mapping of snapshot ID to the segments filename that is being
     *          snapshotted. The expected input would be the output of
     *          {@link #getSnapshots()}. A null value signals that there are no
     *          initial snapshots to maintain.
     */
    public SnapshotDeletionPolicy(IndexDeletionPolicy primary, Map<String, String> snapshotsInfo) {
        this(primary);

        if (snapshotsInfo != null) {
            // Add the ID->segmentIDs here - the actual IndexCommits will be
            // reconciled on the call to onInit()
            for (Entry<String, String> e : snapshotsInfo.entrySet()) {
                registerSnapshotInfo(e.getKey(), e.getValue(), null);
            }
        }
    }

    /**
     * Checks if the given id is already used by another snapshot, and throws
     * {@link IllegalStateException} if it is.
     */
    protected void checkSnapshotted(String id) {
        if (isSnapshotted(id)) {
            throw new IllegalStateException("Snapshot ID " + id + " is already used - must be unique");
        }
    }

    /** Registers the given snapshot information. */
    protected void registerSnapshotInfo(String id, String segment, IndexCommit commit) {
        idToSnapshot.put(id, new SnapshotInfo(id, segment, commit));
        Set<String> ids = segmentsFileToIDs.get(segment);
        if (ids == null) {
            ids = new HashSet<String>();
            segmentsFileToIDs.put(segment, ids);
        }
        ids.add(id);
    }

    protected List<IndexCommit> wrapCommits(List<? extends IndexCommit> commits) {
        List<IndexCommit> wrappedCommits = new ArrayList<IndexCommit>(commits.size());
        for (IndexCommit ic : commits) {
            wrappedCommits.add(new SnapshotCommitPoint(ic));
        }
        return wrappedCommits;
    }

    /**
     * Get a snapshotted IndexCommit by ID. The IndexCommit can then be used to
     * open an IndexReader on a specific commit point, or rollback the index by
     * opening an IndexWriter with the IndexCommit specified in its
     * {@link IndexWriterConfig}.
     * 
     * @param id
     *          a unique identifier of the commit that was snapshotted.
     * @throws IllegalStateException
     *           if no snapshot exists by the specified ID.
     * @return The {@link IndexCommit} for this particular snapshot.
     */
    public synchronized IndexCommit getSnapshot(String id) {
        SnapshotInfo snapshotInfo = idToSnapshot.get(id);
        if (snapshotInfo == null) {
            throw new IllegalStateException("No snapshot exists by ID: " + id);
        }
        return snapshotInfo.commit;
    }

    /**
     * Get all the snapshots in a map of snapshot IDs to the segments they
     * 'cover.' This can be passed to
     * {@link #SnapshotDeletionPolicy(IndexDeletionPolicy, Map)} in order to
     * initialize snapshots at construction.
     */
    public synchronized Map<String, String> getSnapshots() {
        Map<String, String> snapshots = new HashMap<String, String>();
        for (Entry<String, SnapshotInfo> e : idToSnapshot.entrySet()) {
            snapshots.put(e.getKey(), e.getValue().segmentsFileName);
        }
        return snapshots;
    }

    /**
     * Returns true if the given ID is already used by a snapshot. You can call
     * this method before {@link #snapshot(String)} if you are not sure whether
     * the ID is already used or not.
     */
    public boolean isSnapshotted(String id) {
        return idToSnapshot.containsKey(id);
    }

    public synchronized void onCommit(List<? extends IndexCommit> commits) throws IOException {
        primary.onCommit(wrapCommits(commits));
        lastCommit = commits.get(commits.size() - 1);
    }

    public synchronized void onInit(List<? extends IndexCommit> commits) throws IOException {
        primary.onInit(wrapCommits(commits));
        lastCommit = commits.get(commits.size() - 1);

        /*
         * Assign snapshotted IndexCommits to their correct snapshot IDs as
         * specified in the constructor.
         */
        for (IndexCommit commit : commits) {
            Set<String> ids = segmentsFileToIDs.get(commit.getSegmentsFileName());
            if (ids != null) {
                for (String id : ids) {
                    idToSnapshot.get(id).commit = commit;
                }
            }
        }

        /*
         * Second, see if there are any instances where a snapshot ID was specified
         * in the constructor but an IndexCommit doesn't exist. In this case, the ID
         * should be removed.
         * 
         * Note: This code is protective for extreme cases where IDs point to
         * non-existent segments. As the constructor should have received its
         * information via a call to getSnapshots(), the data should be well-formed.
         */
        // Find lost snapshots
        ArrayList<String> idsToRemove = null;
        for (Entry<String, SnapshotInfo> e : idToSnapshot.entrySet()) {
            if (e.getValue().commit == null) {
                if (idsToRemove == null) {
                    idsToRemove = new ArrayList<String>();
                }
                idsToRemove.add(e.getKey());
            }
        }
        // Finally, remove those 'lost' snapshots.
        if (idsToRemove != null) {
            for (String id : idsToRemove) {
                SnapshotInfo info = idToSnapshot.remove(id);
                segmentsFileToIDs.remove(info.segmentsFileName);
            }
        }
    }

    /**
     * Release a snapshotted commit by ID.
     * 
     * @param id
     *          a unique identifier of the commit that is un-snapshotted.
     * @throws IllegalStateException
     *           if no snapshot exists by this ID.
     */
    public synchronized void release(String id) throws IOException {
        SnapshotInfo info = idToSnapshot.remove(id);
        if (info == null) {
            throw new IllegalStateException("Snapshot doesn't exist: " + id);
        }
        Set<String> ids = segmentsFileToIDs.get(info.segmentsFileName);
        if (ids != null) {
            ids.remove(id);
            if (ids.size() == 0) {
                segmentsFileToIDs.remove(info.segmentsFileName);
            }
        }
    }

    /**
     * Snapshots the last commit. Once a commit is 'snapshotted,' it is protected
     * from deletion (as long as this {@link IndexDeletionPolicy} is used). The
     * commit can be removed by calling {@link #release(String)} using the same ID
     * parameter followed by a call to {@link IndexWriter#deleteUnusedFiles()}.
     * <p>
     * <b>NOTE:</b> ID must be unique in the system. If the same ID is used twice,
     * an {@link IllegalStateException} is thrown.
     * <p>
     * <b>NOTE:</b> while the snapshot is held, the files it references will not
     * be deleted, which will consume additional disk space in your index. If you
     * take a snapshot at a particularly bad time (say just before you call
     * forceMerge) then in the worst case this could consume an extra 1X of your
     * total index size, until you release the snapshot.
     * 
     * @param id
     *          a unique identifier of the commit that is being snapshotted.
     * @throws IllegalStateException
     *           if either there is no 'last commit' to snapshot, or if the
     *           parameter 'ID' refers to an already snapshotted commit.
     * @return the {@link IndexCommit} that was snapshotted.
     */
    public synchronized IndexCommit snapshot(String id) throws IOException {
        if (lastCommit == null) {
            // no commit exists. Really shouldn't happen, but might be if SDP is
            // accessed before onInit or onCommit were called.
            throw new IllegalStateException("No index commit to snapshot");
        }

        // Can't use the same snapshot ID twice...
        checkSnapshotted(id);

        registerSnapshotInfo(id, lastCommit.getSegmentsFileName(), lastCommit);
        return lastCommit;
    }

}
